Piezoelectric resonant elements utilizing thickness-longitudinal vibrations of piezoelectric bodies, thickness sliding vibrations, and so on have high Q values as resonators, and are widely used for oscillating circuits, filter circuits, and so on.
The piezoelectric resonant elements have a large number of resonant peaks having different frequencies based on vibration modes different in their electrical characteristics.
When piezoelectric resonant elements in which spuriouses (frequency responses due to vibrations other than desired vibrations) are not suppressed are used for oscillating circuits, oscillatory frequencies are liable to be shifted into frequencies at which spuriouses exist against the intentions. On the other hand, when the piezoelectric resonant elements are used for filter circuits, out-of-band attenuations are liable to be insufficient.
Accordingly, in the piezoelectric resonant elements, the point of improving electrical characteristics is how to suppress spuriouses. Therefore, various considerations have been conventionally made.
As conventional piezoelectric resonant elements devised for suppression of spuriouses, one having a configuration shown in FIG. 13 has been known, for example.
A piezoelectric resonant element 101 has a piezoelectric substrate 102 polarized in a thickness direction (corresponding to a direction perpendicular to the paper surface), has circular oscillatory electrodes 103 and 104 formed at central parts on upper and lower surfaces of the piezoelectric substrate 102 (FIG. 13 shows the upper surface) such that they are opposed to each other with the piezoelectric substrate 102 sandwiched therebetween, and utilizes a third harmonic wave of thickness-longitudinal vibration. Furthermore, in the above-mentioned piezoelectric resonant element 101, the diameter of the oscillatory electrode 103 on the upper surface and the diameter of the oscillatory electrode 104 on the lower surface are made to differ from each other. This causes a fundamental wave of thickness-longitudinal vibration to leak out to thereby suppress spurious caused by the vibration.    [Patent document 1] Japanese Unexamined Patent Publication No. 2000-134060